Optical multi-wavelength signal transmissions are widely used for communication through optical networks. One example of such transmission is the Wavelength Division Multiplex (WDM) transmission where signals are transported by a plurality of optical carriers. In certain types of such transmission, some or all of individual optical carriers may transport signals comprising multiple streams that can be separated into distinct multiple sub-streams, or constituents, for detection. Even in the absence of sub-streams during the generation and the transport process of the signals, sub-streams can be generated for example at the receiving side to benefit from differential detection schemes.
Herein, the term constituent is meant to refer to a part of information transported by at least one optical wavelength; a plurality of constituents may form part or the totality of the total amount of information carried by a specific wavelength or multiple wavelengths. Examples of such constituents are the amplitude modulated signal of a demodulated QPSK signal and its complementary copy, or the in-phase and the quadrature-phase constituents of a differential quadrature-phase shift keying (DQPSK) signal, or the two orthogonal polarization constituents of a polarization-multiplexed signal. Those skilled in the art would realize that a constituent may not always be present within the original incoming signal, but it may be generated at certain steps of the transmission process. For example the complementary constituent of the QPSK signal is not explicitly present in the original signal and along the transmission line but it may be generated at a filtering step before reception.
Also, herein, any reference to a “separable stream” is to be understood to refer to a stream of wavelengths capable of being distinguished and separated into constituents by at least one category, for example by the type of modulation or by the polarization state, whereas reference to “differential detection” (also known as “balanced detection”) is to be understood to refer to a detection technique based upon the concomitant detection of the signal and its complementary so that the overall detection sensitivity may be improved.
One example of such separable multiple wavelength streams is based upon DQPSK modulation format, which is used for high speed transmission, for example in optical communication networks. Another example of such separable multiple wavelength streams relies upon polarization multiplexed WDM signals. A further example may be a combination of the aforementioned DQPSK and polarization multiplexed WDM signals. One example of said multiple wavelengths where the wavelengths comprise constituents that can be detected according to differential detection schemes is a WDM signal modulated with quadrature-phase shift keying (QPSK) format. Also a combination of the aforementioned QPSK and polarization multiplexed WDM signals may be possible. Signals based upon coherent transmission formats may also be considered to comprise constituents that can be detected according to differential detection schemes.
The demultiplexing of multiple wavelengths into their constituents is in occasions typically performed by using a plurality of discrete components, each component typically dedicated to an individual optical carrier, which has somehow to be individually controlled in frequency, amplitude, phase, etc. for best detection results. This requires a plurality of separate control circuits in order to tune thermally, optically, or electrically, etc., all the demultiplexing elements.